It is known to construct fuel tanks by bonding two separate tank halves to one another to form a single piece fuel tank. A plurality of openings are typically formed in the fuel tank to permit assembly into and communication with various components inside the fuel tank, such as a fuel pump, electrical connectors, vent valves, and the like. As shown in FIG. 1, prior to bonding the two tank halves 1,2 to one another, each half has been thermoformed out of multi-layer polymeric sheets 4,5, with a barrier layer 3a, 3b between the pair of the sheets 4,5. A typical joint for these thermoformed tanks is formed by sealing together outwardly extending flanges 6,7 of each half 1,2 of the tank.
Because the barrier layers 3a, 3b are sandwiched between the sheets 4,5, when the flanges 6,7 are joined there is a space between the barrier layers 3a and 3b that defines a permeation path. The permeation path between the two barrier layers 3a, 3b allows fuel vapors to escape through the joint formed by the mating flanges 6,7 into the atmosphere. In addition, the bonded joint creates a pair of 90° corners at the flanges, thus creating a stress point at the bonded joint of the two halves.